1. Field of the Invention
This invention relates generally to acoustic ranging, and, more particularly, to acoustic ranging using velocity dependent extended spread-spectrum codes.
2. Description of the Related Art
Underwater seismic exploration is widely used to locate and/or survey subterranean geological formations for hydrocarbon deposits. A survey typically involves deploying one or more seismic sources and one or more seismic sensors at predetermined locations. For example, a seismic cable including an array of seismic sensors may be deployed on the sea floor and a seismic source may be towed along the ocean's surface by a survey vessel. The seismic sources generate acoustic waves that travel to the geological formations, where they are reflected and propagate back to the seismic sensors. The seismic sensors receive the reflected waves, which are then processed to generate seismic data. Analysis of the seismic data may indicate probable locations of geological formations and hydrocarbon deposits.
The accuracy of the seismic analysis may be limited by uncertainties in the seismic source and sensor positions. The positions of deployed seismic sources and seismic sensors may be estimated using modelling techniques that predict the position of the deployed seismic sources. For example, the position of a seismic cable on the sea floor may be estimated using models that consider the physical characteristics of the seismic cable (e.g., weight, diameter, etc.) and the effect of predicted sea currents on the seismic cable as it descends to the sea floor. However, such methods are predicated on a limited knowledge of the properties of water in the catenary, as well as the geology of the sea floor, and thus they only provide an estimate of the seismic cable's location.
A variety of measurement techniques have been developed to determine the position of the seismic sources and the seismic sensors as the seismic sensors descend through the catenary and come to rest on the sea floor. One such technique is time delay estimation, which determines the positions of arrays of seismic sources and seismic sensors by measuring the time it takes for a signal, such as a chirp, to travel between the seismic sources and seismic sensors. For example, an acoustic source may be deployed on a buoy at the sea surface. One or more receivers may be deployed along a seismic cable resting on the sea floor. The distance between the acoustic source and the receivers, and, consequently, the position of the seismic cable, may be determined by cross-correlating a positioning signal emitted by the acoustic source with the positioning signal received by the receivers. The cross-correlation produces a peak in the cross-correlation estimate that corresponds to a time lag caused by propagation of the positioning signal from the acoustic source to the receivers.
The ocean's surface, however, is not an ideal platform for the acoustic sources and/or receivers that are used in time delay estimation. Movement of the acoustic source and/or receiver as it rides the rough sea surface may introduce Doppler shifts into the positioning signal, which may degrade the cross-correlation estimates. Even moderately heavy seas with a significant wave height (SWH) of about 8 meters may accelerate a buoy or vessel to velocities of about 2-3 meters per second. The resulting Doppler shift may destroy the peak in the cross-correlation estimate in up to 60% of the attempted measurements. Similarly, the motion of the seismic cable may degrade the cross-correlation estimates, making it difficult to determine the location of the seismic cable as it descends through the catenary to the sea floor. For example, FIG. 1 shows a model cross correlation estimate 10 calculated with stationary sources and sensors. A peak 20 is evident at a time lag of zero. A second correlation estimate 30 at various time lags is calculated including a Doppler shift, and a peak 40 is evident at a non-zero time lag. The amplitude of the peak 40 is reduced relative to the peak 20 because of the Doppler shift.